A method of said type is disclosed by WO 02/37904. With the prior art method the current through the lamp and the voltage across the lamp is measured, sampled and converted to digital values. At each sample time a multiplier multiplies the current value and the voltage value to provide a value for the power which the lamp consumes at that time. The calculated power value is compared with a reference value to provide a first error signal. The first control voltage value is memorized. A second memorized control voltage value was memorized at a preceding sample time. Depending on the signs and the magnitudes of the first and second control voltages a multiplication factor is determined. The first error signal and the product of the second control voltage and said factor are added to provide a combined control voltage. The combined control voltage is used to control a frequency or duty cycle of a high frequency square wave voltage which is supplied to the lamp is operated, such that the first error signal is minimized. The use of said combined control voltage instead of the first control voltage only is to make the control loop stable and at the same time to provide a comparatively quick response for a very wide range of parameters, such as power consumed by the lamp or the ambient temperature (see document page 7 lines 19-21).
Said advantage of quick response turns out to be a disadvantage for a lamp which is operated in a so-called scanning mode, such as in LCD television sets. In that case an LCD display of a television set comprises several longitudinal fluorescent tube lamps, with each lamp being turned on and off alternately to prevent that a viewer experiences a smearing effect on images shown by the display. As an example, the refresh rate of the lamps may be 75 Hz with an on-time of 3 ms and an off-time of 10 ms. This means that each lamp must be re-ignited every 1/75 second. Usually, an ignition voltage and an ignition frequency of the high frequency voltage supplied to a lamp is considerably higher than during normal operation. In more detail, as the error signal becomes larger, the high frequency of the voltage supplied to the lamp is increased. At the time of turning on of the lamp the measured current is zero and thus the calculated power is zero and the error is maximum. As a consequence the power taken by the lamp during ignition is greater than during normal operation. In addition, sudden on switching of the lamp may easily cause overshoots of the power applied to the lamp. In turn this causes a heavy electron bombardment of the electrodes and therefor it reduces the lifetime of the lamp. Said disadvantages are more significant. As a whole said disadvantages will reduce the lifetime of the lamp to an extent which is unacceptable for use in television sets.